Recently, it is needed that semiconductor memory devices, which can operate at a low power supply voltage of, for example, 3.3 volts, dissipate low power and move quickly (have high operation speed). It is assumed that an internal power supply voltage generating circuit well known in the art is incorporated in the semiconductor memory devices supplied with the low power supply voltage and that either of a bipolar transistor and a MOS transistor is used as a driver circuit, particularly a word line driver (125, refer to FIG. 1).
Under this assumption, when the bipolar transistor is used as the word line driver, a word line connected thereto is supplied with an operating voltage dropped by a base-emitter voltage Vbe of the bipolar transistor. That is, the word line may be driven under the operating voltage. When the MOS transistor is used as the word line driver, an operation of the semiconductor memory device becomes instable owing to its low driving capacity.
Generally, in the semiconductor memory devices operating at the low power supply voltage, there is directly employed an external power supply voltage EVC as its operating voltage (that is, a power supply voltage).
However, when the external power supply voltage EVC is directly used as the operating voltage of the prior semiconductor memory device, there may be arisen problems to be described below if the external power supply voltage EVC is increased. Since the external power supply voltage thus increased is immediately supplied on the word line through the word line driver, a potential on the word line becomes higher than a required voltage, so that access transistors 104 and 105 (refer to FIG. 1), which are coupled to the word line, become much turned on. This makes current (or cell current) corresponding to a voltage thus changed flow onto either of bit lines BL and BLB (refer to FIG. 1) (this means that there is consumed more power than before the external power supply voltage is increased). As a result, signal width on each bit line becomes larger, so that a write recovery time t.sub.WR of AC characteristics of the conventional semiconductor memory device becomes longer (this means that its operation speed is reduced). Accordingly, when the external power supply voltage is directly applied to the word line driver, the aforementioned problems are arisen because of an external power supply voltage variation.